A reflective member can be used as a reflector in a vehicle head lamp requiring high luminous intensity or an extension reflector which is a decorative member arranged inside a lamp chamber so as to surround a reflector. Such a reflective member can have aluminum vapor deposition treatment performed on a surface of a base material made of synthetic resin. In the aluminum vapor deposition reflective surface, a constant, high, regular reflectance of about 90% is obtained in all the wavelength ranges, so that the reflective member is widely used in other vehicle lamps in addition to the head lamp.
However, in the aluminum vapor deposition reflective surface, there is still a loss of about 10% in the regular reflectance. Therefore, more improvement in the regular reflectance is desirable.
After a silver vapor deposition film having a high regular reflectance (99%) was developed as a reflective surface for an indoor lighting apparatus, application to a reflective surface of a reflective member of a lamp was examined. However, the silver vapor deposition film reacts by making contact with sulfur dioxide (e.g., present in sweat, exhaust gas), oxygen (hot oxygen) and moisture in the atmosphere (silver oxide or silver sulfide is generated), and the silver vapor deposition film is easily discolored (e.g., yellowed) or corroded, and the decrease in the regular reflectance can be significant.
Hence, it has been proposed to provide a topcoat layer 3 or an undercoat layer 4 of a modified silicon resin with good gas barrier properties at high temperature. The topcoat or undercoat layer is laminated and formed of a silver vapor deposition film 2 on a surface of a base material 1 made of synthetic resin. The topcoat layer 3 or the undercoat layer 4 functions as a gas barrier to sulfur dioxide (sweat, exhaust gas), oxygen and moisture in the atmosphere. Corrosion or discoloration of the silver vapor deposition film 2 is inhibited, and a high, regular reflectance is maintained. (See Japanese Patent Document JP-A-2000-106017 and see FIG. 10).
However, as disclosed by JP-A-2000-106017, using gas barrier properties of the topcoat layer or the undercoat layer (hereinafter called “coat layers”) formed of the modified silicon resin, there was some effect in the case of inhibiting discoloration (yellowing) of a silver vapor deposition reflective surface (silver vapor deposition film), but there were problems in which discoloration or corrosion occurs, and the regular reflectance decreases after a long time (e.g., after a heat resistance test of 400 hours).
Examination led to the discovery that a cause of discoloration (yellowing) is that the gas (moisture, oxygen or sulfur dioxide) in the atmosphere described above makes contact with Ag atoms, and that the discoloration results from the fact that “Ag atoms constructing a silver vapor deposition film vibrate (move) and cohere by heat energy.”
As shown in FIG. 10, the Ag atoms (crystal grains of Ag) that form the silver vapor deposition film on the surface of the base material cohere in places from the original orderly state, and fine unevenness is formed on the surface of the silver vapor deposition film. In a region in which this fine unevenness is formed, light of a short wavelength band (e.g., blue) is absorbed, and light of a long wavelength band (e.g., yellow to red) is reflected, so that the whole silver vapor deposition film appears yellow.
On the other hand, in JP-A-2000-106017, a problem arises because a protective film forming each of the coat layers peels off the interface with the silver vapor deposition film or a crack occurs on a surface of the protective film when using the protective film under high temperature and high humidity (in an environment of, for example, 50° C. and 95%) or keeping the protective film for a long time without being used under high temperature and high humidity.
Examination by the inventor found that a resin component of the modified silicon resin forming the protective film in accordance with JP-A-2000-106017 is composed only of a silicon resin. The protective film with good heat resistance and gas barrier properties is formed, but when used in an environment of high temperature and high humidity, problems result from an increase in internal stress of the protective film by hardening of a siloxane bond (unreacted portion at the time of baking) of the resin component. Hence, it was discovered that the resin component is changed from the silicon resin to other resins and certain flexibility is given to the protective film. Thus, internal stress occurring in the protective film is decreased, and occurrence of a crack or peeling of the protective film is prevented.
However, when a crosslink density of resin forming the protective film is low, while maintaining flexibility, it was found that in addition to the problem of gas barrier properties, Ag atoms (which receive heat energy and cohere to form fine unevenness on a surface of a silver vapor deposition film) pass through the inside of a topcoat layer and cohere so as to protrude from a surface of the topcoat layer and form fine unevenness (hereinafter called a “migration phenomenon”). Therefore, it was discovered that the Ag atoms make contact with sulfur dioxide, for example, on the surface of the topcoat layer so that silver sulfide, for example, is generated. The result is a phenomenon in which the whole silver vapor deposition film appears yellow as a result of a reflection state of light in a cohesion region of the surface of the topcoat layer.
Repeats of experiment and consideration by the inventor revealed that when a silver vapor deposition film is formed of a silver alloy including Nd rather than pure silver, cohesion is significantly inhibited when the silver vapor deposition film and Ag atoms receive heat energy. Therefore, fine unevenness is not formed on the surface of the silver vapor deposition film.
on the other hand, it was discovered that when a transparent modified silicone series resin (having a silicone resin and an acrylic resin as resin components) is used in a protective film of a topcoat layer covering a silver vapor deposition film, sufficient flexibility, migration resistance and gas barrier properties are ensured. Thus, there is no peeling of the protective film from the interface with the silver vapor deposition film, no occurrence of fine unevenness due to cohesion of Ag atoms on a surface of the topcoat layer, and no occurrence of silver sulfide as a result of contact between the silver vapor deposition film and sulfur dioxide. Therefore, the silver vapor deposition film is not discolored (e.g., yellowed), so that there is no decrease in the regular reflectance as a result of discoloration (yellowing) of the silver vapor deposition film.